The induction of apoptosis by the Myc oncoprotein is considered to be a major innate mechanism preventing the emergence of neoplasia. However, although many, diverse proteins have each been proposed as key downstream apoptotic effectors, the molecular mechanisms linking Myc with the basal the apoptotic machinery are largely unknown. Myc sensitizes cells to apoptosis induced by a wide range of insults, suggesting that Myc acts to prime some central determinant of apoptotic decision-making. Consistent with this, our work has directly implicated the mitochondrion as rapid and immediate target of Myc pro-apoptotic action. Based upon this observation, we have used a biased proteomic approach to identify Bcl-xL-interacting proteins that rapidly change in abundance or mobility following acute Myc activation in human and rodent fibroblasts. Three candidates have been identified - the known pro-apoptotic BH3 protein Bim, a novel protein with a consensus BH3 domain called PDCD10 recently shown to be encoded by a Myc target gene, and the small G protein Rab7. We propose to validate each of these candidate Myc apoptotic effectors by a combination of molecular, cell and molecular biological and genetics strategies, both in vitro and in several tissues in vivo using a unique set of switchable Myc mouse transgenic models. Finally, we propose to extend our initial proteomic study to establish a map of the proteomic changes in mitochondria that rapidly accompany Myc activation, with particular emphasis on the restricted proteins in the outer mitochondrial compartment most likely targeted by Myc apoptotic effectors. These studies will provide new insights into the molecular machinery linking deregulated proliferation with its innate tumor suppressive apoptotic failsafe, and how that linkage is severed in cancer cells.